1. Field of The Invention
The present invention relates to a method for distinguishing between at least two targets with the aid of a pulse Doppler radar, with the radar signals reflected from at least one target being evaluated with respect to their distance direction and/or their velocity direction whereupon a conclusion is drawn as to the existence of one or a plurality of targets.
2. Background Information
The term pulse Doppler radar employed in this application also includes the terms pulse Doppler radar system and pulse Doppler radar device.
A pulse Doppler radar can be constructed in a space-saving, compact manner. It is therefore usable in many ways as a socalled on-board radar in land vehicles and aircraft. For such uses it is desirable to have a transmit/receive antenna which has the smallest possible surface area (aperture). However, such a system is able only to accurately determine either the distance or the velocity of a target, for example a land vehicle or an aircraft. If distance and velocity of a target are to be determined simultaneously, this is possible only at the expense of the otherwise attainable maximum accuracies. These facts correspond to the determination that, if the distance and velocity of a target are determined simultaneously with the aid of a pulse Doppler radar, the resolution of the latter in the distance direction and in the velocity direction is reduced with respect to corresponding individual measurements. Therefore, resolution limits exist in the distance direction and in the velocity direction below which multiple targets cannot be discovered. The resolution limit in the distance direction or in the velocity direction is characterized in that, with a certain probability, two targets at the same Doppler velocity can be distinguished separately in the distance direction or, at the same distance, in the velocity direction.
The most important values for the resolution limits of a pulse Doppler radar are, among others, the pulse repetition frequency PRF, the length of the Fast Fourier transform N.sub.FFT employed, the sampling time T.sub.a and the length of the transmitted pulse T.sub.rx. With presently customary methods, for example, the so-called "valley sensing" method, in which a relative minimum between two maxima is located in the signal characteristic as the resolution criterion, resolution values are obtained with a probability of 50% which correspond approximately to 1.5 to 2 times the quantization steps in the measurement scale. In these methods, only the amplitude of the signal characteristic is employed to resolve two targets.